As is known, many carboxylic acids and/or carboxylic anhydrides are prepared industrially by catalytic gas-phase oxidation of aromatic hydrocarbons such as benzene, o-, m-, or p-xylene, naphthalene, toluene or durene (1,2,4,5-tetramethylbenzene) in fixed-bed reactors, preferably multitube reactors. Depending on the starting material, this method is used to produce, for example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellitic anhydride. The customary procedure in such a process is to pass a mixture of a gas comprising molecular oxygen, for example air, and the starting material to be oxidized through a multiplicity of tubes arranged in a reactor, in which tubes a bed of at least one catalyst is located. To regulate the temperature, the tubes are surrounded by a heat transfer medium, for example a salt melt. Despite this thermostating, hot spots in which the temperature is higher than in the remainder of the catalyst bed can occur. These hot spots give rise to secondary reactions such as total combustion of the starting material or lead to the formation of undesirable by-products which can be separated from the reaction product only with great difficulty, if at all, for example the formation of phthalide or benzoic acid in the preparation of phthalic anhydride (PA) from o-xylene.
To reduce the intensity of these hot spots, it has become customary in industry to arrange catalysts of differing activity in zones in the catalyst bed, with the less active catalyst generally being located in the fixed bed such that the reaction gas mixture comes into contact with it first, i.e. it is at the gas inlet end of the bed, while the more active catalyst is located toward the gas outlet end of the catalyst bed (DE-A 25 462 68, EP-A 28 64 48, DE-A 29 48 163, EP-A 16 32 31, U.S. Pat. No. 4,665,200). The catalysts of differing activity in the catalyst bed can be exposed to the reaction gas at the same temperature, but the two zones of catalysts of differing activity can also be thermostated to different reaction temperatures for contact with the reaction gas (DE-A 28 30 765). According to EP-A 16 32 31, a plurality of the measures mentioned can be employed at the same time for achieving the activity structuring described. German Patent Application No. P 19 823 262 describes a variant using a plurality of catalysts in which the activity of the catalysts increases pseudocontinuously from the gas inlet end to the gas outlet end.
To minimize contamination by troublesome color-imparting components such as phthalide or naphthoquinone and thus obtain a PA of good quality and also to avoid contamination of the waste gas by residual xylene or naphthalene, the reaction is carried out at full conversion (i.e. >99.9% conversion based on the starting material used) (K. Towae et al. in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A20, 1992, 181). A comprehensive review of the prior art for the selective oxidation of o-xylene and a description of the process and catalyst production may be found in WO 98/37967 and in K. Towae et. al., loc. cit.
EP-A 256 352 describes a particular process variant for preparing PA, in which o-xylene is first oxidized in the liquid phase using molecular oxygen over a homogeneously dissolved cobalt catalyst to give toluric acid and the toluric acid formed is subsequently oxidized further to PA in the gas phase over a conventional heterogeneous catalyst.
Catalysts which have been found to be useful for these oxidation reactions are coated catalysts in which the catalytically active composition is applied in the form of a shell to a nonporous support material which is generally inert under the reaction conditions, for example quartz (SiO2), porcelain, magnesium oxide, in dioxide, silicon carbide, rutile, alumina (Al2O3), aluminum silicate, magnesium silicate (steatite), zirconium silicate or cerium silicate or a mixture of these support materials. The catalytically active constituents of the catalytically active composition of these coated catalysts are generally titanium dioxide in the form of its anatase modification and vanadium pentoxide. In addition, the catalytically active composition may further comprise small amounts of many other oxidic compounds which, as promoters, influence the activity and selectivity of the catalyst, for example by decreasing or increasing its activity. Examples of such promoters are the alkali metal oxides, in particular lithium, potassium, rubidium and cesium oxide, thallium(I) oxide, aluminum oxide, zirconium oxide, iron oxide, nickel oxide, cobalt oxide, manganese oxide, tin oxide, silver oxide, copper oxide, chromium oxide, molybdenum oxide, tungsten oxide, iridium oxide, tantalum oxide, niobium oxide, arsenic oxide, antimony oxide, cerium oxide and phosphorus pentoxide. Promoters which reduce the activity and increase the selectivity are, for example, the alkaline metal oxides, while oxidic phosphorus compounds, in particular phosphorus pentoxide, increase the activity of the catalyst but reduce its selectivity.
EP-A 447 267 concerns a conventional V2O5—TiO2 (anatase) catalyst for preparing phthalic anhydride; this catalyst can further comprise small amounts of silver in addition to other doping components.
Although the processes for the oxidation of aromatic hydrocarbons to form carboxylic acids and/or carboxylic anhydrides, in particular the oxidation of o-xylene and/or naphthalene to PA, have been studied very intensively for decades, there is still a need for improved catalysts for this purpose.
Silver-vanadium oxide compounds having an atomic Ag/V ratio of <1 are known as silver-vanadium oxide bronzes. These are generally semiconducting or metallically conductive oxidic solids which preferably have layer or tunnel structures in which part of the vanadium is present in reduced form as V(IV) in the [V2O5]∞ host lattice. α-AgxV2O5 bronzes have an orthorhombic crystal structure. They comprise partially reduced [V2O5]∞ layers parallel to the (001) plane which comprise edge- and corner-linked VO5 pyramids. The Ag cations are intercalated between the partially reduced [V2O5]∞ layers. The β-AgxV2O5 bronzes in which x=0.3−0.4 have tunnel structures. The parent β-[V2O5]∞ host lattice is built up of greatly distorted VO6 octahedra and distorted trigonal-bipyramidal VO5 units with formation of large channels. The Ag cations are present in the channels of the β-[V2O5]∞ host lattice. In contrast, the idealized structure of the vanadium bronze δ-AgxV2O5 (x=0.6−0.9) comprises layers of edge-linked VO6 octahedra between which the Ag cations are intercalated.
Further information on the composition and crystal structure of the oxidic bronzes may be found in A. F. Wells, Structural Inorganic Chemistry, Fifth Edition, Clarendon Press, Oxford, 1984, pp. 621-625 and in C. N. R. Rao, B. Raveau, Transition Metal Oxides, VCH Publishers, Inc., New York, 1995, pages 176-179. Specific information on the preparation and structure of the AgxV2O5 bronzes is given in “Gmelin Handbuch der anorganischen Chemie”, 8th edition, silver, part B4, System No. 61, Springer-Verlag, Berlin-Heidelberg-New York, 1974, pp. 274-277.
EP-A 856490 discloses a specific silver-vanadium oxide and its use as cathode material in electrochemical cells. This silver-vanadium oxide is produced in a solid-state reaction between silver oxide and a vanadium oxide such as V2O5 or V6O13, from 500° C. to 520° C.
The use of silver-vanadium oxide bronzes as oxidation catalyst is also known. Thus, Y. I. Andreikov, A. A. Lyapkin and V. L. Volkov in Neftekhimiya 17, 559 (1977) describe the use of Ag—V2O5 bronzes having an Ag:V2O5 molar ratio of 0.8:1 for the oxidation of toluene to benzaldehyde/benzoic acid. Here, the selectivity to desired products decreases with increasing conversion. These catalysts are obtained by joint melting of the starting materials silver or silver nitrate and V2O5 at 750° C., giving a 3-phase mixture which, owing to its method of preparation, has a low BET surface area. In addition, these catalysts may further comprise copper. In RU patent 2088 567, Y. I. Andreikov et al. use Ag—V2O5 bronzes of the above composition on various support materials for the oxidation of toluene to benzaldehyde and benzoic acid. According to the examples, the highest conversion is obtained when using a catalyst comprising the Ag—V2O5 bronze in the form of a shell on a silicon nitride support material. Here, the conversion of toluene into benzaldehyde and benzoic acid at 420° C. is, overall, less than 15%. These catalysts are therefore not economical in operation.
Furthermore, E. I. Andreikov and V. Volkov in Kinet. Katal. 22, 963 (1981) and 22, 1207 (1981) describe the selective oxidation of o-xylene or naphthalene using Ag—V2O5 bronzes having an Ag:V2O5 molar ratio of 0-1:1, with a maximum in respect of activity/selectivity occurring in the range 0.5-0.86:1. In this reaction too, the desired product selectivity decreases with increasing conversion. The catalysts described in these publications are likewise obtained by joint melting of the starting materials.
In addition, JP-A 46-42883 (1971) discloses the oxidation of o-xylene to phthalic anhydride using Ag—V2O5-containing catalysts having an Ag:V2O5 molar ratio of 0.01-1:1 with addition of Tl in a Tl:V2O5 molar ratio of 0.01-1:1. Although high conversions are achieved using this system, the desired product selectivity and yield are unsatisfactory. These catalysts are produced by impregnation of the support material and subsequent drying and calcination.
JP-A 44-29045 (1969) describes the oxidation of isobutene to methacrolein by means of silver vanadate catalysts in which the Ag/V atomic ratio is ≧1.
Finally, the partial gas-phase oxidation of toluene using silver-vanadium oxide bronzes is known from U.S. Pat. No. 3,485,876, DE-A 12 94 951 and U.S. Pat. No. 4,137,259. The Ag:V atomic ratio in these catalysts is 1:1. The partial gas-phase oxidation of cyclopentadiene over Ag—V2O5 (with a V:Ag atomic ratio of 1:0.003) is likewise known (K.-W. Jun et al., Appl. Catal 63, 267-278 (1990)), where the Ag—V2O5 catalysts contain only V2O5 and no other identifiable solid phases. The selective oxidation of noncyclic, unsaturated hydrocarbons, in particular the oxidation of 1,3-butadiene to furan, with the aid of silver vanadates is described in DE-A 19705326.
In all cases, the selectivity and yield for producing the desired products was unsatisfactory, so that industrial use of the silver-vanadium oxide bronzes was of no economic interest.